Cr-Mn-N austenitic stainless steel

ABSTRACT

A Cr—Mn—N austenitic stainless steel, in which moderate manganese (Mn) and nitrogen (N) are essentially substituted for the costly nickel to produce a novel Cr—Mn—N steel, is provided, whereby reducing the cost of materials while maintaining the original physical and mechanical properties. The composition thereof includes by weight: 0.005% to 0.08% carbon, 0.3% to 0.9% silicon, 12.1% to 14.8% manganese, 0.001% to 0.04% phosphorus, 0.001% to 0.03% sulfur, 16% to 19% chromium, 0.5% to 1.8% nickel, 0.2% to 0.45% nitrogen, 0.001% to 0.3% molybdenum, 0.001% to 0.3% copper, and trace elements unavoidable in most manufacturing processes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an austenitic stainless steel, inparticular to an austenitic stainless steel in which manganese andnitrogen are used in substitution for nickel.

2. Related Art

The ordinary stainless steel has the properties of the pleasing whiteluster on the surface and the stainless tendency. There is a variety ofstainless steel that is popular among the consumers and widely used in,for example, stainless steel kitchenware, water tank, mechanicalcomponents, sports gear, aerospace materials, medical instruments, and3C industry etc., in which the most widely and frequently used is the304 stainless steel. The standard composition thereof includes 18%chromium plus 8% nickel i.e., the commonly called 18-8 stainless steel.The characteristics of such stainless steel include good mechanicalproperties, magnetism free, stable metallographic grain structure unableto be changed by heat treatment, good durability, good processability,and superior corrosion resistance due to the higher content of nickel.However, the 304 stainless steel is at a stiff price because of theworldwide shortage of nickel caused by war. Accordingly, it is animportant issue to reduce the nickel content in the aforementioned Cr—Nistainless steel and to use other elements in the composition thereof tomaintain or even enhance the inherent mechanical properties andcorrosion resistance, whereby saving the resource of nickel and reducingthe cost of materials.

SUMMARY OF THE INVENTION

In view of the foregoing, an object of the present invention is toprovide a novel steel of single austenitic structure with less nickel,and the corrosion resistance, strength, elongation of which in themarine atmosphere and the acid atmosphere are at the same level as oreven better than those of the 304 stainless steel.

To solve the aforementioned problem, the present invention disposes atechnique using moderate manganese (Mn) and nitrogen (N) in substitutionfor the costly nickel to produce a novel Cr—Mn—N steel, wherebyproviding a Cr—Mn—N austenitic stainless steel comprising: 0.005% to0.08% carbon by weight; 0.3% to 0.9% silicon by weight; 12.1% to 14.8%manganese by weight; 0.001% to 0.04% phosphorus by weight; 0.001% to0.03% sulfur by weight; 16% to 19% chromium by weight; 0.5% to 1.8%nickel by weight; 0.2% to 0.45% nitrogen by weight; 0.001% to 0.3%molybdenum by weight; 0.001% to 0.3% copper by weight; and traceelements unavoidable in most manufacturing processes.

The effects obtained by practice of the present invention lie in: thepresent invention employs the formation mechanism of austenitic (or γ)steel, substituting moderate manganese and nitrogen for the costlynickel to produce a novel Cr—Mn—N steel of single austenitic structure,while maintaining the corrosion resistance, strength, elongation thereofin the marine atmosphere and the acid atmosphere at the same level as oreven better than those of the 304 stainless steel, so as to achieve thepurpose of reducing the cost of materials. The present invention adoptsthe method of substituting manganese and nitrogen for nickel to producethe pure magnetism-free austenitic stainless steel, the mechanicalproperty UTS of which is approximately 200 MPa higher than that of the304 stainless steel, the Y.S of which is approximately one time higherthan that of the 304 stainless steel, the elongation reaches 50%, andthe corrosion resistance is equal. And the most important is that theunit price thereof is less than half of the 304 stainless steel. Thecharacteristics of this novel steel include excellent fluidity, superiorcasting properties, and good resistance to high temperature oxidation.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given herein below for illustration only, and whichthus is not limitative of the present invention, and wherein:

FIG. 1 is a Schaeffler diagram showing the Ni—Cr equivalent of theCr—Mn—N austenitic stainless steel according to one embodiment of thepresent invention; and

FIGS. 2A and 2B are metallographs showing different parts of the Cr—Mn—Naustenitic stainless steel according to one embodiment of the presentinvention.

DETAILED DESCRIPTION OF HE INVENTION

The contents of the present invention are described in details throughspecific embodiments with reference to the figures. The referencenumerals mentioned in the specification correspond to equivalentreference numerals in the figures.

The present invention essentially includes a Cr—Mn—N stainless steel ofaustenitic metallographic structure, which is melted by electric arcfurnace or vacuum induction furnace. The composition of the Cr—Mn—Naustenitic stainless steel includes by weight: 0.005% to 0.08% carbon;0.3% to 0.9% silicon; 12.1% to 14.8% manganese; 0.001% to 0.04%phosphorus; 0.001% to 0.03% sulfur, 16% to 19% chromium; 0.5% to 1.8%nickel; 0.2% to 0.45% nitrogen; 0.001% to 0.3% molybdenum; 0.001% to0.3% copper, and trace elements unavoidable in most manufacturingprocesses.

The equations of the composition of the above elements are:

Ni equivalent=% Ni+30×% C+0.5×% Mn+30% N

Cr equivalent=% Cr+% Mo+1.5×% Si+0.5×% Cb

Referring to the phase diagram shown in FIG. 1, where the ordinate is Niequivalent and the abscissa is Cr equivalent, if the point falls in theaustenitic area according to computation, then the requirements in table1 are satisfied.

TABLE 1 Composition % C % Si % Mn % P % S % Cr % Ni % N % Mo % Cu MassPercentage 0.002 0.3 to 12.1 to <0.04 <0.03 16 to 0.5 to 0.2 to <0.3<0.3 to 0.08 0.9 14.8 19 1.8 0.45 Maximum 0.002 0.3 12.1 16 0.5 0.1 0Minimum 0.08 0.9 14.8 19 1.3 0.45 0.3

Minimum Ni equivalent=0.5+30×0.002+0.5×12.1+30×0.2=12.61

Maximum Ni equivalent=1.8+30×0.08+0.5×14.8+30×0.45=25.1

Minimum Cr equivalent=16+0+1.5×0.3+0.5×0=16.45

Maximum Cr equivalent 19+0.3+1.5×0.9+0.5×0=20.65

The shadowed area in FIG. 1 shows the major austenitic composition.

Composition analysis of the test samples is shown in table 2.

TABLE 2 Composition % % C % Si % Mn % P % S % Cr % Ni % N Mass 0.00790.65 12.27 17.06 1.68 0.42 Percentage

Maximum Ni equivalent=1.68+30×0.0079+0.5×12.27+30×0.42=20.652

Minimum Cr equivalent=17.06+0+1.5×0.65+0.5×0=18.035

Further referring to the phase diagram shown in FIG. 1, the point fallsin the austenitic area, which satisfies the requirements.

The present invention essentially employs manganese and nitrogen inpartial or complete substitution for nickel. The characteristics ofmanganese and nitrogen are analyzed below.

The influence of manganese on grain structure includes:

a. The content of manganese should be 2% or less when used asdeoxidizer.

b. The content of manganese may be up to 20% when used as an alloyelement.

c. The substitution of manganese for nickel increases solubility ofnitrogen, achieving the effects of saving nickel and enhancing strength.

The influence of manganese on mechanical properties includes:

a. When the content of manganese is 2% or less, hardness is not affectedbut tensile strength and yield strength decrease.

b. The high-temperature thermoplasticity of high Ni—Cr γS.S is improved.

The influence of manganese on corrosion resistance includes:

MnS inclusion causes decreases in corrosion resistance and interstitialcorrodibility

The influence of nitrogen on grain structure includes:

a. Nitrogen dramatically forms and broadens the γ phase area, therebyenhancing the γ stability.

b. Carbide precipitation is suppressed and the precipitation of a phaseis delayed, which benefits the anti-sensitization of intergranularcorrosion and toughness of the steel.

The influence of nitrogen on mechanical properties includes:

a. By means of solid solution strengthening (which forms interstitialsolid solution), strength of the steel significantly increases whileplasticity and toughness decrease.

b. Excess of nitrogen (≧0.84%) results in plastic-brittle transition.

According to the Schaeffler diagram of the Ni—Cr equivalent in FIG. 1,manganese and nitrogen may partially or completely substitute for nickelin γ stainless steel, whereby enhancing strength and maintainingelongation as that of the 304 stainless steel without changing thestructure of the steel.

Harmful elements such as phosphorus and sulfur tend to be generated bythe melted iron in the furnace during smelting, where the content ofphosphorus should be controlled under 0.04% or less, and the content ofsulfur should be controlled under 0.04% or less.

The compositions of the embodiments of the present invention and thecontrast material are shown in table 3 below.

TABLE 3 Specimens Composition Content % Element C Si Mn P S Cr Ni N MoCu 304 Stainless <0.08 <1.0 <2.0 <0.045 <0.03 17 to 8 to 10 / <0.6 /Steel 19 (Contrast Material) Embodiment 1 0.0642 0.69 12.43 0.031 0.01216.87 1.21 0.45 0.026 0.106 Embodiment 2 0.0547 0.81 13.92 0.01 0.00116.71 0.82 0.24 0.025 0.104 Embodiment 3 0.0432 0.85 12.12 0.012 0.00516.38 0.5 0.35 0.027 0.109

The mechanical properties in the aforementioned embodiments in table 3are shown in table 4 below.

TABLE 4 Mechanical Property Density σ_(b) σ_(s) δ Hardness P Salt SprayTest Specimens Mpa Mpa % HRB HB kg/cm³ 36H 48H 304 Stainless Steel 520206 40-50 ≦90 ≦187 7.85 No rust No rust (Contrast Material) Embodiment 1700 400 40-50 86-92 ≦187 7.77 No rust No rust Embodiment 2 751 425.951.5 89 180 7.76 No rust No rust Embodiment 3 706.4 426.7 47.6 88 1757.77 No rust No rust

Referring to the metallographs of different parts shown in FIGS. 2A and2B, it is observed that the structure thereof is γ before heattreatment. Therefore, the steel is a completely γ stainless steel.

To sum up, the present invention is not restricted to the particulardetails described herein. Indeed, those skilled in the art having thebenefit of this disclosure will appreciate that many other variationsfrom the foregoing description and drawings may be made within the scopeof the present invention. Accordingly, it is the following claimsincluding any amendments thereto that define the scope of the invention.

1. A Cr—Mn—N austenitic stainless steel, comprising: 0.005% to 0.08%carbon by weight; 0.3% to 0.9% silicon by weight; 12.1% to 14.8%manganese by weight; 0.001% to 0.04% phosphorus by weight; 0.001% to0.03% sulfur by weight; 16% to 19% chromium by weight; 0.5% to 1.8%nickel by weight; 0.2% to 0.45% nitrogen by weight; 0.001% to 0.3%molybdenum by weight; 0.001% to 0.3% copper by weight; and traceelements unavoidable in most manufacturing processes.
 2. The Cr—Mn—Naustenitic stainless steel according to claim 1, comprising: 0.0642%carbon by weight; 0.69% silicon by weight; 12.43% manganese by weight;0.031% phosphorus by weight; 0.012% sulfur by weight; 16.87% chromium byweight; 1.21% nickel by weight; 0.45% nitrogen by weight; 0.026%molybdenum by weight; 0.106% copper by weight; and trace elementsunavoidable in most manufacturing processes.
 3. The Cr—Mn—N austeniticstainless steel according to claim 1, comprising: 0.0547% carbon byweight; 0.81% silicon by weight; 13.92% manganese by weight; 0.01%phosphorus by weight; 0.001% sulfur by weight; 16.71% chromium byweight; 0.82% nickel by weight; 0.24% nitrogen by weight; 0.025%molybdenum by weight; 0.104% copper by weight; and trace elementsunavoidable in most manufacturing processes.
 4. The Cr—Mn—N austeniticstainless steel according to claim 1, comprising: 0.0432% carbon byweight; 0.85% silicon by weight; 12.12% manganese by weight; 0.012%phosphorus by weight; 0.005% sulfur by weight; 16.38% chromium byweight; 0.5% nickel by weight; 0.35% nitrogen by weight; 0.027%molybdenum by weight; 0.109% copper by weight; and trace elementsunavoidable in most manufacturing processes.